nixos-rebuild: support sudo + --target-host

This adds support for deploying to remote hosts without being root:

  sudo nixos-rebuild --target-host non-root@host

Without this change, only root@host is able to deploy.

The idea is that if the local command is run with sudo, so should the
remote one, thus there is no need for adding any CLI options.

nixos/modules/installer/tools/nixos-rebuild.sh

@@ -22,6 +22,7 @@ repair=
 profile=/nix/var/nix/profiles/system
 buildHost=
 targetHost=
+maybeSudo=
 
 while [ "$#" -gt 0 ]; do
     i="$1"; shift 1
@@ -96,6 +97,9 @@ while [ "$#" -gt 0 ]; do
     esac
 done
 
+if [ -n "$SUDO_USER" ]; then
+    maybeSudo="sudo "
+fi
 
 if [ -z "$buildHost" -a -n "$targetHost" ]; then
     buildHost="$targetHost"
@@ -111,9 +115,9 @@ buildHostCmd() {
     if [ -z "$buildHost" ]; then
         "$@"
     elif [ -n "$remoteNix" ]; then
-        ssh $SSHOPTS "$buildHost" env PATH="$remoteNix:$PATH" "$@"
+        ssh $SSHOPTS "$buildHost" env PATH="$remoteNix:$PATH" "$maybeSudo$@"
     else
-        ssh $SSHOPTS "$buildHost" "$@"
+        ssh $SSHOPTS "$buildHost" "$maybeSudo$@"
     fi
 }
 
@@ -121,7 +125,7 @@ targetHostCmd() {
     if [ -z "$targetHost" ]; then
         "$@"
     else
-        ssh $SSHOPTS "$targetHost" "$@"
+        ssh $SSHOPTS "$targetHost" "$maybeSudo$@"
     fi
 }